[llvm] [CMake] Fix the check for Windows vs ccache (PR #137238)

[Kajetan Puchalski via llvm-commits]

mrkajetanp wrote:

> It could check that as well, but `CMAKE_HOST_WIN32` is a predefined cmake variable that should be equal to `CMAKE_HOST_SYSTEM_NAME STREQUAL "Windows"`, just a bit more convenient.

Fair enough, works the same either way so I suppose it's just down to stylistic preference.

https://github.com/llvm/llvm-project/pull/137238